.\"	$OpenBSD: BIO_should_retry.3,v 1.4 2016/11/20 01:39:11 schwarze Exp $
.\"	OpenSSL 60e24554 Apr 6 14:45:18 2010 +0000
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.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: November 20 2016 $
.Dt BIO_SHOULD_RETRY 3
.Os
.Sh NAME
.Nm BIO_should_retry ,
.Nm BIO_should_read ,
.Nm BIO_should_write ,
.Nm BIO_should_io_special ,
.Nm BIO_retry_type ,
.Nm BIO_should_retry ,
.Nm BIO_get_retry_BIO ,
.Nm BIO_get_retry_reason
.Nd BIO retry functions
.Sh SYNOPSIS
.In openssl/bio.h
.Ft int
.Fo BIO_should_read
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_should_write
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_should_io_special
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_retry_type
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_should_retry
.Fa "BIO *b"
.Fc
.Fd #define BIO_FLAGS_READ			0x01
.Fd #define BIO_FLAGS_WRITE			0x02
.Fd #define BIO_FLAGS_IO_SPECIAL		0x04
.Fd #define BIO_FLAGS_RWS \e
.Fd \&	(BIO_FLAGS_READ|BIO_FLAGS_WRITE|BIO_FLAGS_IO_SPECIAL)
.Fd #define BIO_FLAGS_SHOULD_RETRY	0x08
.Ft BIO *
.Fo BIO_get_retry_BIO
.Fa "BIO *bio"
.Fa "int *reason"
.Fc
.Ft int
.Fo BIO_get_retry_reason
.Fa "BIO *bio"
.Fc
.Sh DESCRIPTION
These functions determine why a BIO is not able to read or write data.
They will typically be called after a failed
.Xr BIO_read 3
or
.Xr BIO_write 3
call.
.Pp
.Fn BIO_should_retry
is true if the call that produced this condition
should be retried at a later time.
.Pp
If
.Fn BIO_should_retry
is false, the cause is an error condition.
.Pp
.Fn BIO_should_read
is true if the cause of the condition is that a BIO needs to read data.
.Pp
.Fn BIO_should_write
is true if the cause of the condition is that a BIO needs to write data.
.Pp
.Fn BIO_should_io_special
is true if some "special" condition
(i.e. a reason other than reading or writing) is the cause of the condition.
.Pp
.Fn BIO_retry_type
returns a mask of the cause of a retry condition consisting of the values
.Dv BIO_FLAGS_READ ,
.Dv BIO_FLAGS_WRITE ,
.Dv BIO_FLAGS_IO_SPECIAL
though current BIO types will only set one of these.
.Pp
.Fn BIO_get_retry_BIO
determines the precise reason for the special condition.
It returns the BIO that caused this condition and if
.Fa reason
is not
.Dv NULL
it contains the reason code.
The meaning of the reason code and the action that should be taken
depends on the type of BIO that resulted in this condition.
.Pp
.Fn BIO_get_retry_reason
returns the reason for a special condition
if passed the relevant BIO, for example as returned by
.Fn BIO_get_retry_BIO .
.Pp
.Fn BIO_should_retry ,
.Fn BIO_should_read ,
.Fn BIO_should_write ,
.Fn BIO_should_io_special ,
and
.Fn BIO_retry_type
are implemented as macros.
.Pp
If
.Fn BIO_should_retry
returns false, then the precise "error condition" depends on
the BIO type that caused it and the return code of the BIO operation.
For example if a call to
.Xr BIO_read 3
on a socket BIO returns 0 and
.Fn BIO_should_retry
is false, then the cause will be that the connection closed.
A similar condition on a file BIO will mean that it has reached EOF.
Some BIO types may place additional information on the error queue.
For more details see the individual BIO type manual pages.
.Pp
If the underlying I/O structure is in a blocking mode,
almost all current BIO types will not request a retry,
because the underlying I/O calls will not.
If the application knows that the BIO type will never
signal a retry then it need not call
.Fn BIO_should_retry
after a failed BIO I/O call.
This is typically done with file BIOs.
.Pp
SSL BIOs are the only current exception to this rule:
they can request a retry even if the underlying I/O structure
is blocking, if a handshake occurs during a call to
.Xr BIO_read 3 .
An application can retry the failed call immediately
or avoid this situation by setting
.Dv SSL_MODE_AUTO_RETRY
on the underlying SSL structure.
.Pp
While an application may retry a failed non-blocking call immediately,
this is likely to be very inefficient because the call will fail
repeatedly until data can be processed or is available.
An application will normally wait until the necessary condition
is satisfied.
How this is done depends on the underlying I/O structure.
.Pp
For example if the cause is ultimately a socket and
.Fn BIO_should_read
is true then a call to
.Xr select 2
may be made to wait until data is available
and then retry the BIO operation.
By combining the retry conditions of several non-blocking BIOs in a single
.Xr select 2
call it is possible to service several BIOs in a single thread,
though the performance may be poor if SSL BIOs are present because
long delays can occur during the initial handshake process.
.Pp
It is possible for a BIO to block indefinitely if the underlying I/O
structure cannot process or return any data.
This depends on the behaviour of the platforms I/O functions.
This is often not desirable: one solution is to use non-blocking I/O
and use a timeout on the
.Xr select 2
(or equivalent) call.
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr BIO_read 3
.Sh BUGS
The OpenSSL ASN.1 functions cannot gracefully deal with non-blocking I/O:
they cannot retry after a partial read or write.
This is usually worked around by only passing the relevant data to ASN.1
functions when the entire structure can be read or written.
